I haven't so much been mistaken as an employee as taken it upon myself to be one (temporarily).
For example:
There are times I will be at Circuit City or the Apple Store and will see potential customers walking or standing around with dazed/confused looks on their faces in front of particular pieces of equipment, with no help in sight. I usually engage them in polite conversation and honestly answer any questions they may have.
Relatedly, I will often overhear Employees at Circuit City (or similar) answer questions from enthusiatic but uneducated customers either in a inane or manipulative manner. I usually take it upon myself to interject with more honest options. I have been thanked on many occasions.
I also tend to buss my own dishes at restaurants and occasionally other people's as well...
I have worked in the service indutry so long, it is like second - nature, even though I don't really work in it anymore...
I guess I just want people to make informed decisions and to be happy.